In large rooms such as school classrooms, lecture halls, auditoriums, theaters and the like, there is often a need for audio amplification of a speaker's remarks. Not only do individuals in the back rows of seats need to hear the speaker clearly, but these audio amplification systems can also be used for other purposes. For example, in today's classrooms many audio systems are used in order to enhance each student's learning experience, including: paging, audio enhancement for television, overhead projectors, and microphone systems for teachers. These audio systems are frequently wireless. This type of system gives the speaker great freedom to walk about the room or stage to work on a blackboard, operate audio-visual equipment and the like. The microphone transmits a wireless signal to a receiver/audio amplifier unit located within the room, and the amplifier sends an amplified signal (usually by wire) to speakers mounted within the room, typically near the back.
Currently, there are two main types of technologies available for wireless audio amplification systems: FM Radio Frequency (RF) systems, and Infrared (IR) systems. Both systems have advantages and disadvantages as described below:
Radio Frequency (RF)
A typical prior art “pure” RF system is shown in FIG. 1. As used herein, “pure” means that only one signal transmission mode is used, in this case RF. In FIG. 1, a teacher in a classroom carries or wears a microphone 102 that broadcasts the teacher's voice by RF signals. The signals can propagate throughout the room, through objects in the room such as blackboards 104, and even beyond the room. The signals are picked up by an RF receiver 108 mounted somewhere in the room, amplified and then sent to wired speakers 110, 112. Outside light, such as from windows 106, does not interfere with signal transmission. Since the signals can penetrate the walls of the classroom, if the teacher leaves the classroom and forgets to turn off the microphone, the teacher's voice will still be heard by students in the room.
Some typical advantages of a “pure” RF system include: (1) The RF signal is normally able to be received from anywhere in the room. (2) The RF signal does not require line of sight contact between transmitter and receiver. Thus, for example, a teacher could walk behind a blackboard and still be heard by the students. (3) Signal strength is usually strong and can be transmitted/received through walls, up to a determined area. (4) Only one receiver is needed because of the signal strength and the ability of the RF signal to bounce/reflect off some objects. (5) The receiver can be placed in any area of the room, and does not need to be in a specified place.
Disadvantages of a “pure” prior art RF system include: (1) An RF Microphone is limited to the number of channels (frequencies) that can be selected. (2) A different channel must be used for each room, and the user must manually set the channel on the microphone to match the receiver's channel in each particular room. (3) The RF signal does not stop at the classroom walls. Thus, the signal can often be inappropriately received in other classrooms or when the teacher leaves the classroom. For example, if the teacher leaves the classroom, does not turn off the microphone and is speaking, the signal is still being received inside the classroom that is set to the same frequency, and the audio voice is being transmitted through the speakers. This becomes a problem when the teacher is speaking and does not want the students in the class to hear his or her ongoing conversation outside the classroom. (4) There must be some synchronization between the microphone and the receiver. (5) Radio interference is often a problem. (6) Students are sometimes able to change the channel without the knowledge or permission of the teacher.
Infrared (IR)
A typical prior art “pure” IR system is shown in FIG. 2. The only signal transmission mode used in this case is IR. A teacher 100 in a classroom carries or wears a microphone 102 that broadcasts the teacher's voice by IR signals. The signals can generally propagate throughout the room, but only by line of sight. They cannot propagate through objects in the room such as blackboards 104, or beyond the room. The signals are picked up by one or more IR receivers 120, 130, 140 mounted within the room, amplified and then sent to wired speakers 110, 112. For large rooms, or if there is bright ambient light, such as near windows 106, then more than one IR receiver may be needed. If ambient light is extremely bright, such as from direct sunlight, then the IR signal could be completely interrupted.
Infrared systems have some advantages. Since IR light cannot go through walls, a facility can install and use the same type of system in multiple rooms without causing interference. Also, when a teacher leaves the receiver's area (such as a classroom), the teacher's microphone will automatically shut down because interrupting the direct line of sight between transmitter and receiver will cause signal contact to be lost. In this way, the teacher does not need to be concerned about making inappropriate remarks outside the classroom that could be heard by students in the classroom.
Disadvantages of a “pure” prior art IR system include: (1) Line of sight contact is required between the transmitter and receiver. Therefore, for example, the receiver must be installed in the ceiling or high on the wall and cannot be covered or obstructed. (2) Even though an IR signal can be reflected off walls, the signal strength may be weak or choppy in areas near windows where there is much light, and in areas behind or near a blackboard which can absorb the light. (3) Since IR light cannot penetrate walls, a classroom with an “L” shape or other unusual shape, or a classroom that has interior partitions, needs multiple receivers that usually must be installed into the ceiling, which can be time-consuming and expensive. (4) Usually the receiver's control panel must be placed in an accessible area, and not mounted in the ceiling with the receiver. (5) IR systems typically require a fair amount of power to transmit the IR signal. Thus, for example, a portable microphone with an IR transmitter needs a large battery, or a number of small batteries, that must be replaced frequently.
Because of the above-mentioned disadvantages of prior art “pure” RF and “pure” IR audio systems, a need exists for a wireless audio amplification system that maximizes the advantages and minimizes the disadvantages of RF and IR systems.